Metal Separator For Fuel Cell and Manufacturing Method Thereof

ABSTRACT

A metallic separator for fuel cells having a metal plate, an electroconductive coating layer covering at least a surface in front and back surfaces of the metal plate which contacts a raw material and/or a reaction product, and an electroconductive channel-forming member disposed on a surface of the coating layer and forming a channel for the raw material and/or the reaction product and/or a channel for a cooling medium for cooling. A surface layer on the metal plate has a tensile residual stress within such a range that no stress-corrosion cracking occurs.

TECHNICAL FIELD

The present invention relates to a metallic separator for fuel cells anda method of manufacturing the metallic separator and, more particularly,to a metallic separator suitable for stacked fuel cells using a polymerelectrolyte membrane, a method of manufacturing the metallic separator,and a fuel cell. The present application is accompanied with claim ofpriority under the Paris convention based on Japanese Patent ApplicationNo. 2005-178036 based on the Japanese Patent Law.

BACKGROUND ART

A polymer electrolyte fuel cell (PEFC) is a device for obtainingelectric energy by causing a hydrogen gas provided as a fuel and oxygenprovided as an oxidizer to react with each other. A unit cell formed asthis fuel cell is constituted by a membrane electrode assembly (MEA)which is formed of a pair of porous electrodes (porous supportinglayer+catalyst layer) are opposed to each other with a polymerelectrolyte membrane interposed therebetween, and which is sandwichedbetween a pair of separators in each of which a channel for supplying afuel or an oxidizer is formed. Unit cells formed in this way are stackedto be used as a stacked-cell battery. Various uses of such fuel cells aspower sources for use on vehicles, fixed use and portable/mobile use atan operating temperature of about 80° C. are being expected. Theelectrode reaction is shown below.

Anode: H₂→2H⁺+2e ⁻

Cathode: 2H⁺+1/2O₂+2e ⁻→H₂O  (Formula 1)

At the anode (fuel pole), a fuel such as hydrogen or alcohol is oxidizedto produce hydrogen ions (protons). The produced protons move in theelectrolyte membrane toward the cathode (oxygen pole or air pole)together with water, while electrons reach the cathode via an externalcircuit. At the cathode, water is produced by reduction reaction ofelectrons and oxygen. At this time, the protons produced at the anodemove with water molecules through the electrolyte membrane and,therefore, the electrolyte membrane is maintained in a wet state. Theseparator is exposed to a strong acid solution atmosphere at atemperature from room temperature to 100° C. because it is in contactwith the porous supporting layer (carbon paper or the like) constitutingthe MEA.

The separator has the current collection function and the functions ofseparately supplying a fuel or an oxidizer and discharging a reactionproduct as well as the function of acting as a mechanical reinforcementat the time of stacking. The separator further has the function ofreleasing or uniformizing heat generated by power generation reaction.

Separator materials are roughly divided into carbon materials andmetallic materials. As carbon materials, a piece of graphite obtained bymachining a graphite block, a carbon resin molded piece, an expandedgraphite molded piece, etc., exist. With such materials, there areproblems described below. A graphite block is high-priced and a largenumber of cutting steps are required for cutting it. A carbon resinmolded piece can crack easily. An expanded graphite molded piece hashigh gas permeability.

On the other hand, a metallic separator has high electric conductivity,thermal conductivity, mechanical strength and hydrogen gasimpermeability. Further, the development mainly of a metallic separatorusing mainly austenitic stainless steel as a promising material on whichmachining for forming a channel for a raw material fluid can be easilyperformed and which therefore enables reducing the manufacturing costand the thickness is being pursued. However, there is a problem that themetallic separator is low in corrosion resistance. That is, theelectrolyte membrane is superacidic and the anode side is put in anoxidizing atmosphere at about 100° C. and the cathode side in a reducingatmosphere, as described above. Also, in the vicinity of the metallicseparator, a reacting material and a reaction product contact and anuneven temperature distribution in the areal direction occurs. Thereforea local cell can form easily in the metallic separator and there is anextremely high risk of the metallic separator being corroded. Also, anacid produced by degradation/decomposition of the electrolyte membrane,for example, during use of the metallic separator in continuousoperation for a long time further increases the possibility ofcorrosion. This acid not only corrodes and damages the metallicseparator but also reduces the electric conductivity of the electrolytemembrane by eluted metal ions. Further, there is a problem that elutedmetal ions are precipitated to reduce the performance of a preciousmetal catalyst such as platinum. It is, therefore, difficult to put themetallic separator into actual use.

As a means for solving these problems, a method of forming anelectroconductive polymer coating on the surface of a metallic separatoror a method of forming a corrosion-resisting metal coating layer such asgold or platinum plating is used. For example, patent document 1discloses a metallic separator having a metallic base member on which acontinuous channel is formed by pressing, and which is coated with acoating layer having high adhesion. According to this document,separation of the coating layer does not occur easily, and corrosion ofthe metallic base member can be prevented.

Patent document 2 discloses a metallic separator having an intermediatemetal layer in which a flow channel can be easily formed by stamping, acorrosion-resisting metal layer provided on the outer surface of theintermediate metal layer, and a coating layer of an electroconductiveagent and a resin binder formed on the surface of thecorrosion-resisting metal layer. According to this document, thecorrosion resistance of the metallic base member can be maintained.

Patent document 3 discloses, as an invention in an earlier applicationmade public after the basic application for the right of priority of thepresent invention, a separator structure in which an electroconductivechanneled plate and a metal plate are combined. Also, in patent document3, the provision of a coating layer for preventing corrosion or forlimiting the growth of a passive film over the entire surface of themetal plate or at least on the portion to be brought into contact with ameandering slot is proposed to prevent corrosion of the separator andreduce the contact resistance.

Patent document 1: Japanese Patent Application Laid-Open No. 2000-243408

Patent document 2: Japanese Patent Application Laid-Open No. 2003-272659

Patent document 3: Japanese Patent Application Laid-Open No. 2005-294155

DISCLOSURE OF THE INVENTION

However, the maintenance of the corrosion resistance during a long timehas not been attained by any of the conventional methods. As a life offuel cells for vehicle use, a life of about 5 years (about 44000 hours)is being expected. Also, as a life of fuel cells for fixed use, a lifeof about 10 years (about 88000 hours) is being expected. Thus, thecorrosion resistance metallic separators have been a serious challengeover a long time. Measurements of the amount of elution of a metal in asulfuric acid solution and electrode potentials have been made as ameans for evaluating the corrosion resistance of a material. However,while it is possible to evaluate entire-surface corrosion, it isdifficult to evaluate local corrosion such as local pitting, grainboundary corrosion or stress-corrosion cracking. In particular, it isdifficult to find a sign of stress-corrosion cracking. In many cases,stress-corrosion cracking is found in a time period of several to fiveyears after a start of use. There has been a difficulty in devisingmeasures against it.

Stress-corrosion cracking is a corrosion phenomenon in which a crack iscaused in a metallic material by interaction of tensile stress and acorrosive environment and grows with time. The cause of occurrence ofthis stress corrosion is concurrence of three factors: 1) a materialfactor (metallic material design), 2) a stress factor (the existence ofresidual stress, the influence of a working hysteresis), and 3) acorrosive environment factor. For example, in the case of stainlesssteel, corrosion occurs when the material is exposed to water containinga halogen or oxygen as a corrosion condition. This is similar to acondition for occurrence of pitting. In some cases, pitting is astarting point of stress-corrosion cracking. In a case where a carbideof chromium is precipitated at a grain boundary in a metallic materialto form a chromium-deficient layer in the vicinity of the crystal grainboundary, grain boundary corrosion may occur and progress to causestress-corrosion cracking. The conventional metallic separators arestamped mainly at the time of forming a channel. Further, the interiorof a fuel cell is in a corrosive environment in which a halogengenerated, for example, in a polymer coating or by decomposition of anelectrolyte membrane, oxygen and water coexist. With metallicseparators, therefore, there is a serious problem that stress-corrosioncracking occurs easily.

As measures against corrosion of a metallic separator, therefore, makingselection of a metal strong against a use environment and coating thematerial with a suitable coating layer for prevention of corrosion areperformed. A practical example of the coating layer is a tight organicpolymer film having electric conductivity. However, electricconductivity and corrosion resistance are in a trade-off relationshipand designing a coating material and improving film making techniquesare one of the important challenges to be achieved.

Conventional metallic separators are stamped in a complicated manner toform a channel. Therefore, humidifying water, reaction product water,and acid produced by degradation/decomposition of the electrolytemembrane and the like stagnate easily, for example, at channel bendingportions and portions of the separator and the porous supporting layerin contact with each other. These materials in the metallic separatorbecome a cause of corrosion of the metallic separator and also become acause of reduction in reaction efficiency. Thus, selection of a methodof forming a channel for reactant gas and a metallic material has beenan important challenge influencing the reactant gas flow distributionand the reaction efficiency (power generation efficiency).

The inventors of the present invention did not succeed in effectivelypreventing the occurrence of stress-corrosion cracking during long-timeuse even in the case of forming the metal plate described in theabove-mentioned patent document 3.

The present invention has been achieved in consideration of thesecircumstances and an object of the present invention is to provide ametallic separator for fuel cells having reduced susceptibility tostress-corrosion cracking, having improved corrosion resistance andcapable of maintaining the desired reaction efficiency, a method ofmanufacturing the metallic separator, and a fuel cell.

To achieve the above-described object, according to a first aspect ofthe present invention, there is provided a metallic separator for fuelcells characterized by including a metal plate, an electroconductivecoating layer covering at least a surface in front and back surfaces ofthe metal plate which contacts a raw material and/or a reaction product,and an electroconductive channel-forming member disposed on a surface ofthe coating layer and forming a channel for the raw material and/or thereaction product and/or a channel for a cooling medium for cooling, themetallic separator being also characterized in that a surface layer onthe metal plate has a tensile residual stress within such a range thatno stress-corrosion cracking occurs.

The inventors first paid attention to reducing stress factors andmaterial factors as a measure against stress-corrosion cracking. Inparticular, tensile residual stress in the surface of the metallic platewhich may act as a stress factor occurs easily, for example, at the timeof working the metal plate, at the time of forming the channel-formingmember and at the time of assembling the cell. Therefore, there is aneed to devise the composition of the coating layer, the forming methodand so on for the purpose of positively reducing tensile residual stressin the surface of the metal plate.

According to this aspect a metal plate in which no groove was formed wasused as a metallic separator base plate. A surface of this metallicplate was covered with a coating layer in the form of a tight polymerthin film by thermocompression. Further, a channel-forming member isseparately disposed on the coating layer to form a channel.

Thus, use of a metal plate as a base plate ensures that the coatinglayer in the form of a tight polymer thin film can be formed bythermocompression such as hot pressing. In this way, the corrosionresistance can be largely improved while maintaining electricalconductivity (reduction of material factors). Also, since achannel-forming member separate from the metallic plate is disposed onthe coating layer, a channel can be formed without requiring strongstamping which may cause metal strain (removal of a stress factor).Thus, the susceptibility to stress-corrosion cracking can be largelyreduced by removing or reducing factors responsible for stress-corrosioncracking. Consequently, a metallic separator capable of being used for along time can be provided.

In this aspect, the channel-forming member is finally formed on thecoating layer and, therefore, the placement and physical properties ofthe channel-forming member can be changed freely. Accordingly, a channelcan be designed by adjusting the porosity, water repellency and otherproperties of the channel-forming member on a place-by-place basis.Therefore, there is no possibility of a channel for a fuel or anoxidizer being stopped by product water, condensed water or the like,and the flow distribution of the fuel or the oxidizer can be uniformizedto improve the reaction efficiency (power generation efficiency). Also,the structure of the metallic separator of the present invention issimple. Therefore, operations for manufacturing the separator,assembling a stacked-cell battery and maintenance are made easier toperform.

A second aspect is characterized in that, in the first aspect, thetensile residual stress in the surface of the metal plate is 15 kg/mm²or less.

In the second aspect, the range of a physical property of the metalplate for limiting stress-corrosion cracking is specified. Solid polymerfuel cells are ordinarily used as a stacked-cell battery in which unitcells are stacked according to an output. Restricting tensile residualstress within the range according to the second aspect ensures thatstress-corrosion cracking can be limited for a long time period even ifstresses received at the time of assembly into a stacked-cell batteryare added together. Tensile residual stress in the surface of the metalplate can be measured by an X-ray stress measuring method (referencedocument: X-ray stress measurement standards (1997 edition) published bythe X-ray material strength department committee of the Society ofMaterials Science, Japan).

A third aspect is characterized in that, in the first or second aspect,the metal plate is a solution-heat-treated austenitic stainless steelplate.

A fourth aspect is characterized in that, in any of the first to thirdaspects, the metal plate is a metal formed of one or more of inconel,nickel, gold, silver and platinum, or an austenitic stainless plateplated or clad with some of these metals.

In the third and fourth aspects, the material of the metal plate isspecified. The corrosion resistance of the metallic separator can beimproved by using the metal according to the third and fourth aspects.In the fourth aspect, inconel is a trade name of a nickel alloy whichcontains chromium, iron, silicon and other elements, and which is aspecial alloy excellent in heat resistance, acid resistance andcorrosion resistance.

A fifth aspect is characterized in that, in any of the first to fourthaspects, the coating layer and/or the channel-forming member contains acarbon-based electroconductive material and/or a polymer resin.

One of the main functions of the metallic separator is the currentcollection function. In the power generation reaction in the fuel cell,however, the possibility of corrosion is high because of direct exposureto strong acid atmosphere. Also, improving the efficiency of reactionrequires prevention of an uneven distribution of heat. It is, therefore,necessary for the metallic separator to simultaneously have highelectric conductivity, corrosion resistance and heat conductivity (heatdissipation ability).

According to the fifth aspect, each of the coating layer covering thesurface of the metallic separator and the channel-forming memberdisposed on the coating layer contains a mixture of a polymer resin anda carbon-based electroconductive material. The carbon-basedelectroconductive material is an electroconductive material excellent incorrosion resistance and also excellent in terms of cost and handling.Thus, high electric conductivity can be imparted to the polymer resin bymixing the carbon-based electroconductive material in the polymer resin.Also, the corrosion resistance of the polymer resin can be improved byadding the carbon-based electroconductive material and the density canbe improved by increasing the content of the polymer resin. In this way,permeation to the metal plate of a material which causes corrosion (anacid solution or the like) can be limited to improve the corrosionresistance. Since the carbon-based electroconductive material is alsoexcellent in heat conductivity, heat conductivity can be imparted to thecoating layer and to the channel-forming member, thereby uniformizingthe distribution of heat in the areal direction.

A sixth aspect is characterized in that, in any of the first to fifthaspects, the carbon-based electroconductive material contained in thecoating layer and the channel-forming member is one or more of graphite,carbon black, diamond-coated carbon black, silicon carbide, titaniumcarbide, carbon fibers and carbon nanotubes.

In the sixth aspect, the kind of the carbon-based electroconductivematerial contained in the coating layer and the channel-forming memberis specified. The coating layer and the channel-forming member havinghigh electric conductivity can be formed by using this carbon-basedelectroconductive material. As carbon black, ketjen black, acetyleneblack, furnace black or the like is preferred. However, these materialsare not exclusively used. Also, electroconductive carbides of metalsother than silicon carbide and titanium carbide can be used.

A seventh aspect is characterized in that, in any of the first to sixthaspects, the polymer resin contained in the coating layer and thechannel-forming member is one or more of a phenolic resin, an epoxyresin, a melamine resin, a rubber resin, a furan resin and apolyvinylidene fluoride resin.

In the seventh aspect, the kind of the polymer resin used in the coatinglayer and the channel-forming member is specified. These resins areexcellent in corrosion resistance and formability and also excellent inmechanical strength after forming. These resins have eitherthermosetting or thermoplastic characteristics, have formingtemperatures proper for operations and cost, are capable of goodadhesion to metals and are, therefore, markedly effective in reducingthe cost and in improving the corrosion resistance.

An eighth aspect is characterized in that, in any of the fifth toseventh aspects, the carbon-based electroconductive material in thecoating layer contains granular carbon and fibrous carbon and the massratio of the granular carbon and the fibrous carbon is within the rangefrom 1:0.5 to 1:1.5.

In ordinary cases, the coating layer has low electric conductivity inthe areal direction while having high electric conductivity in thethickness direction. In such cases, a current collection distributionoccurs in the areal direction, such that the possibility of occurrenceof nonuniformity of reaction and nonuniformity of distribution of heatis increased, resulting in a reduction in reaction efficiency.

In the eighth aspect, therefore, granular carbon and fibrous carbon aremixed in the polymer resin to form electric conduction paths both in thethickness and areal direction. In this way, the electric conductivityand the corrosion resistance can be improved both in the thicknessdirection and in the areal direction of the coating layer to improve thereaction efficiency. As granular carbon in the eighth aspect, a graphitepowder or carbon black having a grain size of 1 μm or less is preferred.Also, as fibrous carbon, PAN-based or pitch-based carbon fiber having alength of 100 μm or less is preferred. However, these granular andfibrous carbons are not exclusively used.

A ninth aspect is characterized in that, in any of the fifth to eighthaspects, the coating layer contains 40 to 65 mass % of the carbon-basedelectroconductive material and the volume resistivity of the coatinglayer is 50 mΩ-cm or less.

In the metallic separator, it is preferable, from the viewpoint ofsecuring high electric conductivity, to minimize the volume resistivityof the coating layer. The metallic separator can be reduced in thicknessin comparison with carbon separators which have been put into actual useand can therefore be put to practical use if the volume resistivity ofthe coating layer is 50 mΩ-cm or less. It is preferable and morepractical to set the volume resistivity of the coating layer to 30 mΩ-cmor less. In ordinary cases, the higher the volume resistivity of thecoating layer, the higher the corrosion resistance. Therefore, thecontent of the resin is preferably 35 mass % or higher, more preferably40 mass % or higher. Thus, both the desired electric conductivity andcorrosion resistance can be ensured by setting the content of thecarbon-based electroconductive material according to the ninth aspect.

A tenth aspect is characterized in that, in any of the first to ninthaspects, the hydrogen gas permeability of the coating layer is 10mLmin⁻¹m⁻² or less.

In the tenth aspect, the density of the coating layer is specified.Permeation to the metal plate of components which cause corrosion (agas, a liquid) can be limited by constructing the coating layer so thatthe permeability is within the range according to the tenth aspect. Thecorrosion resistance of the metallic separator can be improved thereby.The gas permeability can be adjusted, for example, by the contents ofthe resin and the carbon-based electroconductive material, the kinds ofthe materials, forming conditions (thermocompression) and other factors.The hydrogen permeability can be measured by passing hydrogen on oneside of a flat sample and analyzing hydrogen permeating to the surfaceon the opposite side with a gas chromatography.

An eleventh aspect is characterized in that in any of the first to tenthaspects, the coating layer is formed by thermocompression including ahot press or a hot roll.

According to the eleventh aspect, the coating layer can be formed byadhering the resin forming the coating layer to the metal plate whileuniformly pressing the resin on the metal plate. At this time, gapsbetween portions of the carbon-based electroconductive material in theresin and gaps between portions of the resin are crushed by the pressureand heat, thereby increasing the density. Thus, permeation of a fuel oran oxidizer and a product or the like to the metal plate can be limitedto improve the corrosion resistance. Further, by thermocompression, athin film is formed with good adhesion to the metal plate. Therefore,the interfacial resistance can be reduced and separation of the coatinglayer can be limited. Also, because of the flat metal plate, pressingcan be performed with improved uniformity along the areal direction, andmetal strain due to pressing can be limited to the minimum degree.Therefore, thermocompression is applicable and a polymer having highercorrosion resistance and electric conductivity and free from defectssuch as pinholes and cracks can be uniformly formed into a thin film incomparison with a coating layer formed by ordinary spray coating or animmersion method.

A twelfth aspect is characterized in that, in the eleventh aspect, thethickness of the coating layer is within the range from 10 to 100 μm.

In the twelfth aspect, the range of the coating layer thickness in whichthe coating layer can be formed with a uniform thickness and corrosionof the flat metal plate can be limited is specified. From considerationof the reliability and safety of the metallic separator, the coatinglayer thickness is preferably 10 to 50 μm, more preferably 15 to 50 μm.

A thirteenth aspect is characterized in that, in any of the fifth totwelfth aspects, the channel-forming member contains 40 to 80 mass % ofthe carbon-based electroconductive material.

In the metallic separator, the channel-forming member forms a channelfor supplying a fuel or an oxidizer or discharging a product anddirectly contacts the MEA to collect a current. According to thethirteenth aspect, the carbon-based electroconductive material havinghigh electric conductivity and corrosion resistance is mixed with thepolymer resin in forming. The channel-forming member having highelectric conductivity and corrosion resistance can be obtained thereby.This channel-forming member is provided in the form of a frame memberwith a height (about 0.3 to 1 mm) according to use by forming in a moldor injection molding. The volume resistivity of the channel-formingmember is preferably 10 mΩ-cm or less. Accordingly, the content of thecarbon-based electroconductive material in the channel-forming member ispreferably 40 mass % or more, more preferably 60 mass % or more, andfurther preferably in the range from 70 to 80 mass %.

Peripheral portions of the metallic separator are used as a manifold forsupplying a fuel or an oxidizer to each cell and a sealing portion forseparating the channel for the fuel or the oxidizer from the outside ofthe cell. Also, a member in which the content of the carbon-basedelectroconductive material is small, about 40 mass % or less, may beformed on a peripheral portion of the metallic separator in the samemanner as described above.

A fourteenth aspect is characterized in that, in any of the first tothirteenth aspects, the channel-forming member is formed on the coatinglayer by injection molding or forming in a mold.

In many cases, during power generation reaction, product water andcondensed water is produced and this product water and condensed waterstop the channel for a fuel or an oxidizer to impede the reaction.According to the fourteenth aspect, the channel-forming member is formedby forming in a mold or injection molding. Therefore the adhesion to thecoating layer can be improved. Also, since the channel-forming member isprovided on the flat plate (on the coating layer), the size accuracy ofthe channel is high and the workability is high.

A fifteenth aspect is characterized in that, in any of the first tofourteenth aspects, the channel is formed by combining thechannel-forming members differing in porosity from each other.

There have been cases where during power generation product water andcondensed water stop the channel to reduce the reaction efficiency. Inparticular, a catalyst layer portion in contact with the channel-formingmember is not smoothly supplied with a fuel or an oxidizer and thereaction cannot occur easily in this portion. Even if the reactionoccurs in the catalyst layer portion in contact with the channel-formingmember, a reaction product or the like is accumulated to cause areduction in performance. According to the fifteenth aspect thechannel-forming member has a porous or gas-permeable structure throughhaving high permeability to water. A portion in which the porosity ofthe channel-forming member is a gas/liquid permeable and diffusiblestructure. In this high-porosity portion, supply of a fuel or anoxidizer is promoted and water locally condensed, if any, can be easilydischarged by a flowing gas. Consequently, the reaction efficiency canbe improved. Also, the placements of the channel-forming members on theanode side and the cathode side can be independently designed. Further,channels can be designed by combining porous members differing inporosity, gas-permeable members and non-porous members as thechannel-forming members. In this way, supply of a fuel or an oxidizer toan electrode, removal of a product and supply of water to theelectrolyte membrane can be easily controlled, thus enablingcontribution to a reduction in size of a battery unit.

A sixteenth aspect is characterized in that, in any of the first tofifteenth aspects, the channel is formed by combining one or more of thechannel-forming member having a porosity of 50% or more, thechannel-forming member having a porosity of 10 to 50% and thechannel-forming member having a porosity of 10% or less.

Since the channel-forming member is a member in direct contact with theMEA, it is necessary for the channel-forming member to have suitableheat conductivity.

In the sixteenth aspect, ranges of porosity enabling simultaneouslyobtaining the electric conductivity, heat dissipation performance andmechanical strength are specified. The channel-forming member is therebyenabled to maintain the electric conductivity, to dissipate heataccompanying power generation by diffusing the heat together with theflowing fuel or oxidizer and to thereby uniformize the distribution ofheat. Since the channel-forming member has pores, the elasticity of thechannel-forming member is improved and the adhesion between the MEA andthe separator can also be improved. Thus, excessive mechanical stressapplied to the MEA and the metal plate can be reduced.

A seventeenth aspect is characterized in that, in the sixteenth aspect,the channel-forming member is one or more of porous carbon-basedelectroconductive materials: carbon particle sintered material, carbonfiber sintered material, carbon fiber woven fabric and carbon fibernonwoven fabric, and the channel-forming member is joined to the coatinglayer.

In the seventeenth aspect, concrete kinds of channel-forming member arespecified. A channel having high electric conductivity and corrosionresistance can be easily formed by using the porous carbon-basedelectroconductive member according to the seventeenth aspect even in acase where the channel-forming member contains no resin. Preferably, theporosity of the porous electroconductive member in the seventeenthaspect is 50% or more. The carbon-based porous material may be joined bya bonding material resin or may be joined only by a mechanical force.

To achieve the above-described object, according to an eighteenth aspectof the present invention, there is provided a metallic separator forfuel cells characterized in that a channel for a raw material and/or areaction product and/or a channel for a cooling medium for cooling isformed by forming on a metal plate an electroconductive coating layercovering at least a surface in front and back surfaces of the metalplate which contacts the raw material and/or the reaction product, andthereafter forming the metal plate on which the coating layer is formed.

According to the eighteenth aspect, a more uniform and tighter coatinglayer can be formed in comparison with the case of forming a channel bydirectly stamping a metal plate and thereafter forming a coating layerthereon. In this way, the occurrence of stress-corrosion cracking underthe influence of stamping can be reduced. Hydroforming is preferred as aforming method.

A nineteenth aspect is characterized in that, in the eighteenth aspect,a tensile residual stress in the surface of the metal plate is 15 kg/mm²or less.

In the nineteenth aspect, tensile residual stress in the surface of themetal plate can be measured by an X-ray stress measuring method(reference document: X-ray stress measurement standards (1997 edition)published by the X-ray material strength department committee of theSociety of Materials Science, Japan).

A twelfth aspect is characterized in that in the eighteenth ornineteenth aspect the metal plate is a solution-heat-treated austeniticstainless steel plate.

A twenty-first aspect is characterized in that, in the eighteenth totwelfth aspects, the metal plate is a metal formed of one or more ofinconel, nickel, gold, silver and platinum, or an austenitic stainlessplate plated or clad with some of these metals.

A twenty-second aspect is characterized in that in any of the eighteenthto twenty-first aspects, the coating layer contains a carbon-basedelectroconductive material and/or a polymer resin.

A twenty-third aspect is characterized in that, in the twenty-secondaspect, the carbon-based electroconductive material in the coating layercontains granular carbon and fibrous carbon and the mass ratio of thegranular carbon and the fibrous carbon is within the range from 1:0.5 to1:1.5.

A twenty-fourth aspect is characterized in that in any of the eighteenthto twenty-third aspects, the hydrogen gas permeability of the coatinglayer is 10 mLmin⁻¹m⁻² or less.

A twenty-fifth aspect is characterized in that, in any of the eighteenthto twenty-fourth aspects, the coating layer is formed bythermocompression including a hot press or a hot roll.

A twenty-sixth aspect is characterized in that, in any of the eighteenthto twenty-fifth aspects, the thickness of the coating layer is withinthe range from 20 to 100 μm.

According to a twenty-seventh aspect, a fuel cell is constructed bysandwiching a cell having a cathode catalyst layer on one surface of apolymer electrolyte membrane and an anode catalyst layer on the othersurface of the polymer electrolyte membrane between a first separator inwhich a channel for flowing a fuel to the anode catalyst layer is formedand a second separator in which a channel for flowing an oxidizer to thecathode catalyst layer is formed, and is characterized in that themetallic separator for fuel cells according to any one of the first totwenty-sixth aspects is used as each of the first separator and thesecond separator.

The fuel cell according to the twenty-seventh aspect is based on anapplication of the metallic separator of the present invention. Thecorrosion resistance of stacked-cell battery can be improved thereby toenable long-time operation. In the twenty-seventh aspect, separatorseach having a channel in one face are used as the first and secondseparators at opposite ends of a stacked-cell battery. At innerpositions other than the opposite ends, the flat faces of a pair ofmetallic separators (opposite from the fuel or oxidizer channelformation surfaces) are opposed to each other and a cell coolingstructure may be inserted therebetween if necessary.

In this cell cooling structure, a channel through which a cooling mediumis caused to flow can be formed, as in the above-described method offorming a channel for a fuel or an oxidizer. With respect to a cell notrequiring a cooling medium channel, it is preferable in terms of cost toform, for the functions of the first and second separators, a bipolarstructure in which an anode channel is formed in one face of one metalplate having two surfaces covered with the coating layer, and in which acathode channel is formed in the other face of the metal plate.

The metallic separator of the present invention can be applied to fuelcells using, as a fuel, hydrogen gas, a modified gas and a liquid fuelsuch as alcohol. For example, it can be applied to a polymer electrolytefuel cell (PEFC) and a direct methanol fuel cell (DMFC). Since themetallic separator is thinner than the conventional carbon separator, itenables a stacked-cell battery to be reduced in size. The cell in theeighteenth aspect represents an MEA in which an anode catalyst layer isjoined to one surface of a polymer electrolyte membrane and a cathodecatalyst layer is joined to the other surface of the polymer electrolytemembrane.

To achieve the above-described object, according to a twenty-eighthaspect of the present invention, there is provided a method ofmanufacturing the metallic separator for fuel cells according to any oneof the first to seventeenth aspects, characterized by forming thechannel for the raw material and/or the reaction product and/or thechannel for the cooling medium for cooling by forming theelectroconductive coating layer on at least a surface in the front andback surfaces of the metal plate which contacts the raw material and/orthe reaction product and by thereafter disposing the one or moreelectroconductive channel-forming members on the surface of the coatinglayer.

In the twenty-eighth aspect, a method of manufacturing the metallicseparator of the present invention is specified to remove or reducefactors responsible for stress-corrosion cracking (by using a metalplate not stamped and forming a coating layer in the form of a tightpolymer thin film on a surface of the metal plate). By this method, ametallic separator in which the susceptibility to stress-corrosioncracking is markedly reduced and which is capable of use for a long timecan be provided.

Disposing the channel-forming member on the coating layer enables thequality and physical properties of the channel-forming member and thechannel design to be changed according to use. Therefore the flowdistribution of a fuel or an oxidizer can be uniformized to improve thereaction efficiency.

The channel for a cooling medium for cooling can be formed in the samemanner as the channel for a fuel or an oxidizer. A cooling mediumchannel may be directly formed in the face opposite from the face inwhich a fuel or oxidizer channel is formed or may be formed in thesurface of another metallic separator. Since the structure of themetallic separator of the present invention is simple, operations formanufacturing the separator, assembling a stacked-cell battery andmaintenance can be easily performed.

A twenty-ninth aspect is characterized in that, in the twenty-eighthaspect, the coating layer is formed by adhering a coating layer formingliquid, a coating layer forming powder or a coating layer forming sheetcontaining a carbon-based electroconductive material and a polymer resinto the surface of the metal plate by thermocompression including a hotpress or a hot roller.

According to the twenty-ninth aspect, it is preferable to form thecoating layer as a tight thin film having a thickness of 10 to 100 μm bya thermocompression using a hot press, or a hot roll or the like forpressing the surface while heating the same. More preferably, thecoating layer is formed to a thickness of 10 to 50 μm. By pressing thecoating layer, carbon fibers or carbon particles are brought into closecontact with each other and aligned together to improve the bondingbetween the carbon fibers or particles. The volume resistivity isthereby reduced. Also, air bubbles in the resin layer, and gaps betweenthe resin and carbon fibers are reduced, thereby obtaining a tightcoating layer. From this, a corrosion prevention effect on the metalplate provided as a base can be expected.

Thermocompression according to the present invention can be adoptedbecause the base plate is flat. Also, a polymer having higher electricconductivity and free from defects such as pinholes can be uniformlyformed into a thin film in comparison with a coating layer formed byordinary spray coating or an immersion method. Therefore, permeation tothe metal plate of a material which causes corrosion can be limited toprevent corrosion. Further, the same compressive stress as that in thecase of shot-peening can be given, so that the susceptibility tostress-corrosion cracking can be reduced.

A thirtieth aspect is characterized in that, in the twenty-eighth ortwenty-ninth aspect, the channel-forming member is formed by forming achannel forming liquid or a channel forming powder containing acarbon-based electroconductive material and a polymer resin on thecoating layer by injection molding or forming in a mold to provide thechannel for the raw material and/or the reaction product and/or thechannel for the cooling medium for cooling.

In the thirtieth aspect, a method of forming the channel-forming memberis specified, thereby enabling formation of a channel with improvedbonding to the coating layer. The channel-forming member may beseparately made and thereafter disposed on the coating layer as desiredby in mechanical contact therewith.

To achieve the above-described object, according to a thirty-firstaspect of the present invention, there is provided a method ofmanufacturing the metallic separator for fuel cells according to any oneof the eighteenth to twenty-sixth aspects, characterized in that thechannel for the raw material and/or the reaction product and/or thechannel for the cooling medium for cooling is formed by forming on themetal plate the electroconductive coating layer covering at least asurface in the front and back surfaces of the metal plate which contactsthe raw material and/or the reaction product, and thereafter forming themetal plate on which the coating layer is formed.

In the thirty-first aspect, hydroforming is preferred as a formingmethod.

A thirty-second aspect is characterized in that, in the thirty-firstaspect, the coating layer is formed by adhering a coating layer formingliquid, a coating layer forming powder or a coating layer forming sheetcontaining a carbon-based electroconductive material and a polymer resinto the surface of the metal plate by thermocompression including a hotpress or a hot roller.

According to the thirty-first and thirty-second aspects, a more uniformand tighter coating layer can be formed in comparison with the case offorming a channel by directly stamping a metal plate and thereafterforming a coating layer thereon. In this way, the occurrence ofstress-corrosion cracking which can be easily caused in a bendingportion by stamping or the like can be reduced.

According to the present invention, as described above, a metallicseparator for fuel cells having reduced susceptibility tostress-corrosion cracking and excellent corrosion resistance can beobtained to enable maintenance of the desired reaction efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) is a partial sectional view of a metallic separator used inopposite end portions of a stacked-cell battery, in metallic separatorsin a first embodiment;

FIG. 1( b) is a partial sectional view of a metallic separator used inan inner portion of a stacked-cell battery, in the metallic separatorsin the first embodiment;

FIG. 1( c) is a partial sectional view of a metallic separator used inan inner portion of a stacked-cell battery, in the metallic separatorsin the first embodiment;

FIG. 1( d) is a partial sectional view of a metallic separator inopposite end portions of a stacked-cell battery, in the metallicseparators in the first embodiment;

FIG. 2( a) is a schematic partial view showing a section of thestructure in a coating layer of the metallic separator in the firstembodiment;

FIG. 2( b) is a schematic partial view showing a section of thestructure in a rib of the metallic separator in the first embodiment;

FIGS. 3( a) to 3(e) are schematic diagrams showing the process ofmanufacturing the metallic separator in the first embodiment;

FIG. 4 is a graph showing a cyclic voltammogram (current-voltage curve)of an epoxy resin/carbon coating layer;

FIG. 5 is a scanning-electron-microscopic image of the epoxyresin/carbon coating layer after CV measurement;

FIG. 6( a) is a partial sectional view of a metallic separator in asecond embodiment in a case where a bending portion in a channel sectionis rounded;

FIG. 6( b) is a partial sectional view of a metallic separator in thesecond embodiment in a case where a bending portion in a channel sectionis not rounded;

FIGS. 7( a) to 7(c) are schematic diagrams showing the process ofmanufacturing the metallic separator in the second embodiment;

FIG. 8 is a perspective view showing the entire construction of apolymer fuel cell according to the present invention;

FIG. 9 is a graph of power generation characteristics in an example ofthe present invention;

FIG. 10( a) is a perspective view showing an anode-side face of ametallic separator for a direct methanol fuel cell according to thepresent invention;

FIG. 10( b) is a perspective view showing a cathode-side face of themetallic separator for a direct methanol fuel cell according to thepresent invention; and

FIG. 10( c) is a schematic diagram showing a the flow of air in thecathode-side face of the metallic separator for a direct methanol fuelcell according to the present invention.

DESCRIPTION OF SYMBOLS

-   10 Metallic separator-   50 Metallic separator-   12 Metal plate-   14 Coating layer-   16 a Rib-   16 b Rib-   16 w Rib-   17 Channel-   24 Polymer resin-   26 Polymer resin-   34 Electroconductive material-   36 Electroconductive material-   40 Hydraulic forming apparatus-   42 Lower mold-   44 Upper mold-   50A Anode-side channel formation face (metallic separator)-   50C Cathode-side channel formation face (metallic separator)-   52 Metal plate-   54 Coating layer-   56 a Nonporous rib-   56 b Gas-permeable rib-   56 c Porous rib-   55 Fuel supply manifold-   55′ Oxidizer supply manifold-   57 Fuel discharge manifold-   57′ Oxidizer discharge manifold-   58 Gasket-   40 MEA-   56 d Porous rib (carbon paper)-   100 Polymer electrolyte fuel cell (PEFC)

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred forms of implementation a metallic separator for fuel cells, amanufacturing method and a fuel cell according to the present inventionwill be described with reference to the accompanying drawings.

First Embodiment

The present embodiment is an example of a metallic separator in which achannel is formed by using a channel-forming member without pressing ametal plate.

The construction of the metallic separator in the present embodimentwill first be outlined. FIG. 1 are schematic sectional views of metallicseparators 10 for fuel cells in several patterns. In FIG. 1, FIGS. 1( a)and 1(d) are partial sectional views of metallic separators 10 used inopposite end portions of a stacked-cell battery. FIGS. 1( b) and 1(c)are partial sectional views of metallic separators 10 used in innerportions of the stacked-cell battery. In these figures, FIG. 1( c) is apartial sectional view of a metallic separator 10 having a coolingstructure. FIG. 2 are partial schematic views for explaining membersconstituting FIG. 1. In the figures, the same reference charactersdenote the same objects or the same functions.

The metallic separator 10 in FIG. 1( a) or 1(d) showing a basicconstruction of the metallic separator according to the presentinvention has a metal plate 12 formed of a metallic material having highelectric conductivity (e.g., stainless steel or a nickel alloy), acoating layer 14 covering one surface of the metal plate 12, and ribs 16a provided as channel-forming members disposed on the coating layer 14to form a channel for a fuel or ribs 16 c provided as channel-formingmembers forming a channel for an oxidizer (ribs 16 a or ribs 16 c in thefollowing). Referring to FIG. 1( a) or 1(d), it is preferable, from theviewpoint of increasing the corrosion resistance, to also form a coatinglayer 14 on the surface on which no ribs 16 a or ribs 16 c are formed.

Similarly, the metallic separator 10 in FIG. 1( b) has coating layers 14covering opposite surfaces of one metal plate 12, ribs 16 a disposed onthe surface of one of the coating layers 14, and ribs 16 c disposed onthe surface of the other coating layer 14. Further, the metallicseparator 10 in FIG. 1( c) has, between a pair of metal plates 12covered with coating layers 14, a cooling structure formed of ribs 16 wforming a channel through which a cooling medium (cooling water or thelike) is caused to flow.

The metal plate 12 is a metal in the form of a plate having a suitablemechanical strength, high electric conductivity and excellent corrosionresistance. As metallic materials having such properties, an austeniticstainless steel plate, a metal formed of at least one of inconel,nickel, gold, silver and platinum and a member plated or clad with someof these metals and other materials may be mentioned. However, the metalplate 12 is not limited to these. The metal plate 12 undergoes asolution heat treatment. Preferably, the metal plate 12 has a tensileresidual stress of 15 kg/mm² or less after the solution heat treatment.The possibility of stress-corrosion cracking resulting from residualstress in the metal plate 12 can be reduced thereby.

The coating layer 14 is a protective layer for preventing corrosion ofthe metal plate. For corrosion resistance, a tight film having nodefects such as pin holes and cracks, having good adhesion to the baseplate and having a constant uniform thickness is required.

FIG. 2( a) is a schematic diagram showing an section in the coatinglayer 14. The coating layer 14 has a matrix of a polymer resin 24 inwhich an electroconductive material 34 in granular and fibrous forms(carbon-based conductive material) is uniformly dispersed. FIG. 2( b) isa schematic diagram showing a section in the ribs 16 a, 16 c, and 16 w.In the ribs 16 a, 16 c, and 16 w, an electroconductive material 36 ingranular form is uniformly dispersed in a matrix of a polymer resin 26,as is that in the coating layer 14. As the polymer resin 26 and theelectroconductive material 36, the same materials as the polymer resin24 and the electroconductive material 34 used in the coating layer 14may be used.

FIG. 2( b) is particularly a schematic structural diagram in a casewhere graphite powder 36 in single granule form and thermoplastic resin26 having excellent flowability are used at the time of forming ribs byinjection molding. In a case where ribs are formed by injection molding,however, the material of the ribs may be the same as that in FIG. 2( a)if only the desired flowability can be secured. In a case where ribs areformed by forming in a mold, and the material of the rib is not limitedby the presence of fluidity.

The polymer resin 24 is mainly for imparting corrosion resistance to thecoating layer 14 and is adhered to the metal plate 12. Morespecifically, one or more of a phenolic resin, an epoxy resin, amelamine resin, a rubber resin, a furan resin and a polyvinylidenefluoride resin can be preferably used.

The electroconductive material 34 is mainly for imparting high electricconductivity to the coating layer 14. In the present embodiment, theelectroconductive material in granular and fibrous forms (carbon-basedelectroconductive material) is uniformly dispersed in the polymer resinto form an electric conduction path. It is effective to use theelectroconductive material in either of granular and fibrous forms.

Preferably, the electroconductive material 34 is a carbon-basedelectroconductive material excellent in terms of electric conductivity,dispersibility, cost and handling. As the carbon-based electroconductivematerial, one or more of graphite, ketjen black, acetylene black,furnace black, carbon black, diamond-coated carbon black, siliconcarbide, titanium carbide, carbon fibers and carbon nanotubes can bepreferably used. As the electroconductive material 34, not only thecarbon-based electroconductive material but also a metal oxide, anitride, a boride or the like can also be used.

The coating layer 14 is formed by plastically deforming and pressing acoating layer forming agent (a coating layer forming solution, a coatinglayer forming powder or a coating layer forming sheet) containing thepolymer resin 24 and the electroconductive material 34 at a temperatureequal to or higher than the softening point of the polymer resin 24 orin a setting temperature range (thermocompression). Concrete examples ofthis thermocompression are hot pressing, hot rolling and the like. Inthis way, by a compression force, carbon fibers or carbon particles arebrought into contact with each other and densely arrayed together toimprove bonding between the carbon fibers or carbon particles, therebyreducing the volume resistivity. Also, air bubbles in the resin layerand gaps between the resin and the carbon fibers or carbon particles arereduced, so that the tight coating layer 14 can be obtained.

The ribs 16 a or 16 c are members forming a channel through which a fuelor an oxidizer is supplied or discharged in the metallic separator 10.Also, the ribs 16 a and 16 c establish electrical connection between theMEA and the coating layer 14 and regulate the flow distribution of thefuel or the oxidizer and product water, condensed water or the like.

The ribs 16 w are members forming a channel through which a coolingmedium (cooling water or the like) is supplied and discharged withefficiency. The ribs 16 w are provided as a cooling structure fortemperature control in a stacked-cell battery in correspondence witheach unit cell or each of groups of unit cells each consisting ofseveral unit cells.

As shown in FIGS. 1( a) to 1(d), the ribs 16 a and 16 c are disposed inthe form of frame members with a height of about 0.3 to 1 mm on thecoating layer 14. Electrical connections and the ribs which regulate theflow distribution of the fuel or the oxidizer and product water,condensed water or the like may coincide with each other or may beprovided separately from each other. For example, electrical connectionsin the form of columns may be disposed uniformly on the entire area, andthe peripheral ribs and the ribs forming an inner gas passage may bearranged like Dashboards separately from the electrical connections. Ifthe base plate of the metallic separator is flat as in the presentinvention, an arrangement setting of ribs can be freely made. Therefore,the degree of freedom with which the rib structure is designed can beincreased and the reaction efficiency can be improved. The presentembodiment has been described with respect to an example in which thechannel sectional shape formed by the ribs 16 a, 16 b, and 16 w isrectangular. However, the present invention is not limited to this. Thechannel may be trapezoidal in section.

Ribs differing in porosity may be provided according to the arrangementof the MEA. Transmission and diffusion of the fuel or the oxidizer andproduct water, condensed water or the like through the ribs are therebyfacilitated to improve the reaction efficiency. Porous ribs having aporosity of 50% or higher are permeable both to a gas and to a liquid.Gas-permeable ribs having a porosity of 10 to 50% are permeable to agas. If the porosity is 5% or less, the heat conductivity (heatdissipation ability) is reduced. Thus, local accumulation of heat can beeffectively avoided by a suitable porosity. It is also effective toincrease the porosity of the ribs 16 a and 16 c on the downstream sidewhere water can condense easily, depending on the arrangement of theMEA. Also, the water repellency of the ribs 16 on the downstream sidemay be increased to facilitate discharge of a mass of water. Also, it iseffective to increase the flow rate by reducing the distance between theribs 16 a or 16 c adjacent to each other on the downstream side infacilitating discharge of water. The reaction efficiency can bemaintained in this way. It is also possible to impart elasticity to theribs 16 a, 16 c, and 16 w by including a foaming agent in the rawmaterial resin. Mechanical impacts applied to the metal plate 12 and theMEA can be reduced by doing so. As another form of use of porous ribs, amethod may be adopted in which the width of a rib is increased and thefuel or the oxidizer and product water, condensed water or the like areforced to flow through the rib from one edge of the rib toward anotheredge of the rib (refer to FIG. 10 described below). Fast supply of thefuel or the oxidizer to the MEA in contact with the rib and fastdischarge of reaction products are thereby enabled to improve thereaction efficiency. Thus, since the ribs are formed on the flat baseplate, the size accuracy is high and the workability and joinability tothe coating layer for manufacture can be improved.

The ribs 16 a, 16 c, and 16 w are formed by forming in a mold orinjection molding performed by flowing a channel forming liquidcontaining the polymer resin 26 and the electroconductive material 36directly onto the coating layers 14 and setting the liquid. In this way,channels can be formed with good joinability to the coating layers 14.The ribs 16 a, 16 c, and 16 w may alternatively be formed separately anddisposed mechanically on the coating layers 14 or by thermocompression(hot pressing or the like).

The process of manufacturing the metallic separator will be describedwith reference to FIGS. 3( a) to 3(e).

First, referring to FIG. 3( a), the metal plate 12 is solution heattreated to be formed into a predetermined shape.

Subsequently, as shown in FIG. 3( b), a coating layer forming agent Ahaving the electroconductive material 34 distributed in the polymerresin 24 is applied in film form on the surface of thesolution-heat-treated metal plate.

Subsequently, as shown in FIG. 3( c), the entire surface of the appliedcoating layer forming agent A is thermocompression-formed with a hotroller 19 to form the coating layer 14 in the form of a tight polymerthin film.

Thereafter, as shown in FIG. 3( d), a channel forming solution B (or achannel forming powder B) having the electroconductive material 36distributed in the polymer resin 26 is poured (or packed) on the surfaceof the coating layer 14 by injection molding or forming in a mold (amold 20 in FIG. 3) and is set.

As shown in FIG. 3( e), the mold 20 is removed to form a channel betweenthe ribs.

The metallic separator 10 excellent in corrosion resistance can bemanufactured by such a simple method and arrangement.

The results of studies on the design of the members: the metal plate 12,the coating layer 14 and the ribs 16 a, 16 c, and 16 w will be describedbelow.

1) Metal Plate (Stress-Corrosion Cracking Susceptibility)

A study of the influence of stamping on two materials: austeniticstainless steels SUS302 and SUS304L for the metal plate 12, havinggeneral versatility and actually used under various atmospheres for along time was planned. Rolled members of SUS302 and SUS304L weresolution heat treated by performing water quenching from 1,100° C. bywhich a solid solution of a carbide is produced. Stress-corrosioncracking susceptibility was thereafter measured.

As test pieces, a test piece A and a test piece B not stamped wereprepared, as shown in Table 1. As test pieces for comparison, a testpiece C and a test piece D in which a groove having a depth of 1 mm anda width of 2 mm was formed by stamping, i.e., pressing with a die, wereprepared.

A testing method was carried out in which the test pieces A to D wereimmersed in a boiling 42-mass % magnesium chloride solution for 10 hoursand the state of corrosion of a surface and a section was examined byobservation with an optical microscope and a scanning electronmicroscope.

TABLE 1 Test Stainless piece base plate Stamping Remark A SUS302 UndoneGrain boundaries were slightly corroded but there was no corrosioncracking. B SUS304L Undone No change (no corrosion) C SUS302 DoneCracking occurred along grain boundaries. D SUS304L Done In-graincracking occurred in the vicinity of ridgeline portions

As shown in Table 1, SUS302 test piece A was slightly corroded at grainboundaries after immersion in the boiling 42-mass % magnesium chloridesolution for 10 hours, but no pitting or cracking was recognized. On theother hand, no changes were recognized in SUS304L test piece B. It wasfound that the corrosion resistance of SUS304L test piece B was higherthan that of SUS302 test piece A. In SUS302 test piece C for comparisonin which a longitudinal groove was formed, grain boundary corrosion andcracking along the grain boundary starting from the grain boundarycorrosion occurred. In SUS304L test piece D for comparison, no grainboundary corrosion occurred but in-grain cracking occurred in thevicinity of bent ridgeline portions. It was thereby found that this testpiece had stress-corrosion cracking susceptibility. The tensile residualstress in the portion where cracking occurred due to stamping was 20kg/mm² or more, and it was confirmed that portions in which the tensileresidual stress was 15 kg/mm² or less did not crack. From these results,it was found that forming of the channel without stamping according tothe present invention made it possible to largely reduce thesusceptibility to stress-corrosion cracking.

2) Coating Layer (Corrosion Resistance)

A study was made on the relationship between the corrosion resistance ofthe coating layer, the thickness of the coating layer, the content ofthe carbon-based electroconductive material and the coating layerforming method.

The corrosion resistance of the coating layer was evaluated by measuringthe state of occurrence of corrosion current through alternating currentimpedance. That is, the coating layer was formed on the surface ofstainless steel and a current-potential curve was measured by cyclicvoltammetry (hereinafter referred to as CV measurement) while blowing inair in 1-mol sulfuric acid solution. The sweep rate was set to 10 mV(vsRHE)/sec and an oxidation-reduction current (corrosion current) inoxide film generated at about 600 mV (vsRHE) was detected. Subsequently,the coating layer surface was observed with a microscope. Thereafter,the metal plate was removed to leave the coating layer in the form of a0.5 mm-thick plate, and the hydrogen gas permeability was directlymeasured with a gas chromatograph.

A study on the corrosion resistance of the polymer layer (epoxy resin inthe present embodiment) in the single state not containing theelectroconductive material was first made. That is, a test of therelationship between the resin layer thickness, the forming method andthe corrosion resistance (corrosion current) was conducted.

In the resin layer formed only by a coating method, the corrosioncurrent did not became 100 μA or less when the thickness of the resinlayer is smaller than 100 μm. On the other hand, in the resin layerformed by hot pressing (thermocompression), no corrosion current wasobserved and only a double-layer charge current to the resin layerflowed when the thickness of the resin layer was 20 μm or more. That is,it was found that in the case where the resin layer was formed by hotpressing, the thickness of the layer when sufficient corrosionresistance was exhibited was smaller than in the case of forming theresin layer only by the coating method. From this, it was found that airbubbles and gaps in the epoxy resin were reduced by hot pressing theresin layer to enable tightly forming the resin layer.

A study on the corrosion resistance in a case where the carbon-basedelectroconductive material was added to the epoxy resin was next made.That is, the corrosion current was measured by the same method as thatdescribed above with respect to the coating layer of the epoxy resin towhich 40 to 70 mass of carbon particles having an average primaryparticle size of 300 nm and an average secondary particle size of about1 μm were added, and which was formed on the stainless steel surface.Test pieces a to e shown in Table 2 were provided. Test piece e in thesetest pieces was prepared for comparison between forming methods by beingapplied with a sponge roll and heated at 180° C. for 30 minutes.

TABLE 2 Amount of Amount of Film Peak corrosion Test resin carbonthickness current piece (mass %) (mass %) (μm) (μA/cm²) Remark a 30 7015~20 >50 Hot pressing b 40 60 10~15 ≈0 Hot pressing c 50 50 10~15 ≈0Hot pressing d 60 40 10~15 ≈0 Hot pressing e 50 50 30~50 >100 Rollapplication

As shown in Table 2, it was found that in the case where the coatinglayer was formed by hot pressing, substantially no corrosion currentflowed and the corrosion resistance was good when the amount of theresin containing carbon particles was in the range from 40 to 60 mass %while the thickness of the coating layer was 10 to 15 μm. This resultshows that the corrosion resistance was two times or more higher thanthat in the above-described case of the resin in the single state. Thus,in the present embodiment, sufficient corrosion resistance is exhibitedwhen the thickness of the carbon particle/epoxy resin coating layer is10 to 50 μm. From consideration of the reliability and safety of theseparator, however, it is preferable to set the thickness to 15 to 50μm.

FIG. 4 shows the results of CV measurement in this study. FIG. 4 showscurrent-potential curves with respect to the test piece b formed by hotpressing and the test piece e formed with the sponge roll.

In the case of the test piece b, the current detected during potentialsweep from 0.1 V to 1 V (vsRHE) was within a restricted range of −500 to20 μA and no corrosion current, water decomposition current or the likewas observed. As a result of close observation of the surface of thetest piece b after the measurement, no change was recognized.

In the case of the test piece e, a current considered due to thedecomposition of the passive film or anodic dissolution was detected atabout 800 mV (vsRHE). After CV measurement, the coating layer surface ofthe coating layer/SUS304 test piece e was observed with a scanningelectron microscope. As a result of this observation, the existence ofpinholes such as shown in FIG. 5 in the surface of the coating layer anda change in color of portions around the pinholes accompanying theelution of the metal were recognized. It was confirmed from this resultthat corrosion occurred in the test piece e.

It was confirmed that as a result of use of hot pressing(thermocompression) sufficient corrosion resistance was exhibited evenwhen the thickness of the coating layer is small.

3) Coating Layer and Ribs (Shape and Kind of Electroconductive Material)

Selections from electroconductive materials for the coating layer (andribs) were next made. As the electric conductivity of the coating layer,the volume resistivity of the coating layer was measured by a planardirection four-terminal method. The volume resistivity was measured byinterposing each test piece (coating film) between two metallic platesand maintaining the test piece in a pressed state and by using analternating current impedance method. It is preferable to minimize thevolume resistivity. However, carbon-based materials were selected byusing a practical value of 30 mΩ-cm or less as a guide. Kinds ofcarbon-based electroconductive material are shown below.

f. Carbon fibers: 1 μm in outside diameter, 150 μm in length

g. Nanotubes: 60 to 80 nm in outside diameter, about 100 μm in length

h. acetylene black: 40 nm in average primary particle size, 1 to 2 μM inaggregate average particle size, 70 m²/g in specific surface area

i. ketjen black: 30 nm in average primary particle size, 1 μM inaggregate average particle size, 1300 m²/g in specific surface area

j. 75 mass % of carbon fibers+25 mass % of acetylene black

k. 75 mass % of carbon fibers+25 mass % of ketjen black

l. Graphite fine powder particles:

Coating films in a single state of 15 to 20 μm were made by using anepoxy resin as the polymer resin, adding the carbon-basedelectroconductive materials of specimens f to k described above andperforming thermocompression. Table 3 shows the amount of addition ofthe carbon-based electroconductive materials when a volume resistivityof 30 mΩ-cm was exhibited and the amount of addition of the carbon-basedelectroconductive materials when a volume resistivity of 20 mΩ-cm wasexhibited.

TABLE 3 Unit: mass % 50 mΩ-cm 30 mΩ-cm 20 mΩ-cm Specimen Addition amountAddition amount Addition amount f 35 40 45 g 40 50 55 h 55 60 70 i 45 5055 j 25 35 35 k 30 35 40 l 65 70 80

As shown in Table 3, in the case where the carbon-basedelectroconductive material prepared by mixing 25 mass % of acetyleneblack or ketjen black with carbon fibers, as in specimens j and k, wasused, a volume resistivity of 30 mΩ-cm or less was obtained when theamount of addition was the smallest, 35 mass %. On the other hand, inthe case where the carbon-based electroconductive material was acetyleneblack or ketjen black in a single state, as represented by specimens hand i, obtaining a volume resistivity of 30 mΩ-cm or less requiredaddition of 50 to 60 mass % of the carbon-based electroconductivematerial. It was found that mixing carbon fibers in the coating layermade it possible to reduce the amount of addition of theelectroconductive material, as described above.

Ketjen black is porous, has a high degree of crystallization, existsmixedly among carbon fibers entwining each other to improve electricalcontacts between the carbon fibers, thereby forming a stable electricconduction path. Since the carbon in fibrous form is oriented along theareal direction of the coating layer, it improves the electricconductivity in the areal direction. Thus, the electric conductivity canbe improved both in the thickness direction and in the areal directionby mixing ketjen black in granular form and fibrous carbon in thecoating layer. In this way, the current density distribution in the MEAsurface can be made more uniform to improve the reaction efficiency.

The coating film formed by applying the same carbon fibers as specimen fwith a sponge roll (application method) and heating at 180° C. for 30minutes exhibited a volume resistivity of 30 mΩ-cm when 50 mass % ofcarbon fibers were added. That is, in the case where the coating film isnot processed by thermocompression, about a 10 mass %-larger amount ofcarbon fibers is required for obtaining a volume resistivity of 30 mΩ-cmor less. It is, therefore, supposed that the electric conductivity islow in this case. Accordingly, it is thought that the coating filmformed by hot rolling has increased electric conduction paths thanks tothe mechanical action (pressing action) at the time of thermocompressionto obtain high electric conductivity in comparison with the coating filmformed with a sponge roll.

If a larger amount of electroconductive material is added to the coatingfilm, the amount of resin is reduced and pinholes are formed to increasethe possibility of corrosion such as that in the test piece e shown inTable 2. The thermocompression method according the present inventionenables the coating film to have high electric conductivity whilelimiting the amount of addition of the electroconductive material(without reducing the amount of the resin) and, therefore, the coatingfilm also becomes excellent in corrosion resistance.

While the present embodiment has been described with respect to thecoating layer, it is also preferred that the ribs also have highelectric conductivity, corrosion resistance and heat conductivity. Thesame material as that of the coating layer may be used for the ribs.However, it is supposed that the effectiveness of the electricconductivity in the areal direction is not as high as that of thecoating layer because the rib width is small in ordinary cases.Therefore, mixing granular carbon and fibrous carbon is not necessarilyrequired. The electric conductivity can be improved by increasing thecontent of carbon-based electroconductive material.

Second Embodiment

The present embodiment is an example of a metallic separator having achannel formed by first forming a coating layer on a surface of a metalplate and thereafter forming the plate molding.

FIG. 6 are partial sectional views of a metallic separator 10′ for fuelcells in the present embodiment. In FIG. 6, FIG. 6( a) shows a casewhere a bent portion in a channel section is rounded and FIG. 6( b)shows a case where a bent portion in a channel section is not rounded.FIG. 7 are schematic diagrams for explaining an example of manufactureof the metallic separator 10′ in the present embodiment. In thediagrams, the same reference characters as those in FIGS. 1 to 5 for thefirst embodiment denote the same objects or functions. The samedescriptions of the same objects or functions as those for the firstembodiment will not be repeated.

The metallic separator 10′ in the present embodiment is formed in thesame manner as the metallic separator in the first embodiment exceptthat, as shown in FIG. 6, a channel is formed not by a channel-formingmember but by forming a metal plate 12 having a coating layer 14 formedon its surface, and that the channel has such a sectional shape thatlocal stress concentration is limited.

As shown in FIG. 6( a), tensile residual stress in the metallicseparator 10′ can be reduced by forming bending portions having curvedsurfaces (rounded). In the metallic separator 10′ shown in FIG. 6, thechannel sectional area can be increased.

As the materials for the metal plate 12 and the coating layer 14constituting this metallic separator 10′, various treatments (e.g., aheat treatment on the metal plate 12), the method of forming the coatinglayer, and so on, the same materials, treatments and method, and so onas those in the first embodiment can be used.

The metallic separator 10′ can be manufactured as described below.

First, the coating layer 14 is formed on the surface of the metal plate12 by the same method as that shown in FIGS. 3( a) to 3(c) for the firstembodiment.

Next, a hydraulic forming apparatus 40 (hydroforming apparatus) isprepared which has, as shown in FIG. 7, a lower mold 42 corresponding tothe inversion of the projecting/recessed shape of a channel, an uppermold 44 set so as to be opposed to the lower mold 42 through the metalplate 12 and a pressing rubber bag 46 in which pressing oil L isinjected to press the metal plate 12 in the space between the lower mold42 and the upper mold 44.

Subsequently, the metal plate 12 is set and fixed between the upper mold44 with the pressing rubber bag 46 and the lower mold 42.

Subsequently, pressing oil L at about 200° C. is injected from injectionpiping 49 provided in the upper mold 44 into the pressing rubber bag 46to bend the metal plate 12 having the coating layer 14 formed on itssurface, thereby forming channels 17 on the opposite sides. Air existingbetween the metal plate 12 and the upper mold 44 is exhausted fromexhaust piping 47 connected to an exhaust pump 45 through a valve 48.

Thus, forming is gradually performed by uniformly applying the pressureof pressing oil L to the entire surface of the metal plate 12, so thatlocal stress concentration can be avoided.

The metallic separator 10′ having the channels 17 formed thereon is thenremoved from the hydraulic forming apparatus 40.

The metallic separator 10′ in the present embodiment can be made by theabove-described method. The method of manufacturing the metallicseparator 10′ in the present embodiment is not limited to that describedabove. Any other method may suffice if local stress concentration can bereduced.

Processing for removing tensile residual stress may be performed aftertaking the metallic separator 10′ out of the hydraulic forming apparatus40.

In the present embodiment, if coating layers are formed on the twosurfaces of the metal plate 12, the same coating layer forming operationas that described above may also be performed on the back surface of themetal plate 12. Also, the coating layers may be simultaneously formedwith two rolls opposed to each other, with the metal plate 12 interposedtherebetween. It is preferable to set the thickness of the coating layerwithin the range from 20 to 100 μm from the viewpoint of reliablyreducing stress-corrosion cracking under the influence of forming of themetal plate 12.

As described above, the tight coating layer is formed on the surface ofthe metal plate before forming of the metal plate. Therefore the coatinglayer can be formed more uniformly and tightly at bending portions in achannel cross section in comparison with the case where the coatinglayer is provided after pressing the metal plate. Further, since thechannel is formed by using the forming method that limits local stressconcentration, the occurrence of stress-corrosion cracking in themetallic separator can be limited.

Examples

Examples of the present invention will be described below. The presentinvention is not limited to the examples described below. A powergeneration test of a polymer-electrolyte fuel cell using the metallicseparator 10 in the first embodiment of the present invention wasperformed.

The construction of the polymer electrolyte fuel cell 100 will first bedescribed. FIG. 8 is a perspective view showing the unit-cellconstruction of a polymer electrolyte fuel cell 100. This fuel cell 100has a structure in which a pair of metallic separators 50 of the presentinvention are placed above and below an MEA (membrane electrodeassembly) 40 so that the MEA is in contact with and tightly pinchedbetween the metallic separators 50. The metallic separator 50 has ananode-side channel formation face 50A formed in its upper face and has acathode-side channel formation face 50C formed in its lower face. Thestructure of the cathode-side channel formation face 50C issubstantially the same as that of the anode-side channel formation face50A. Therefore the illustration and description of it are partiallyomitted. The members indicated by reference characters having the sameend numerals are identical to each other or have the same functions.

In the anode-side channel formation face 50A of the metallic separator50, the surface of a metal plate 52 is covered with a coating layer 54and a plurality of ribs are provided on the coating layer 54 to fromchannels. The ribs include a nonporous rib 56 a impermeable to agas/liquid such as a fuel or an oxidizer, a gas-permeable rib 56 bpermeable to a gas, and a porous rib 56 c permeable to a gas/liquid. Theanode-side channel formation face 50A of the metallic separator 50 issealed at its peripheral end with a gasket 58 made of silicon resin toprevent a fuel, an oxidizer or the like from leaking out of the system.Hydrogen gas supplied from a fuel supply manifold 55 flows in thedirections of arrows to flow into an upper portion on the MEA 40 anodecatalyst layer side. The remainder passes through the central porous rib56 c to be recovered through a fuel discharge manifold 57.

On the other hand, in the cathode-side channel formation face 50C of themetallic separator 50, the elements are formed in the same manner asthose described above; the surface of the metal plate 52 is covered witha coating layer and a plurality of ribs (not shown) are provided on thecoating layer to from channels. Oxygen gas entering from an oxidizersupply manifold 55′ flows in channels formed by the ribs to be suppliedto the MEA 40 cathode catalyst layer side, and a reaction product andthe remainder are discharged through an oxidizer discharge manifold 57′.

Conditions for making each member will next be described.

The metal plate 52 was formed by stamping a 0.1 mm-thick SUS304L plateinto a predetermined shape and was heat-treated in a reducing furnace ata temperature of 1,100° C. A material for the coating layer 54 preparedby adding 30 mass % of ketjen black and 25 mass % of carbon fibers to 45mass % of a high-temperature-setting-type epoxy resin was sufficientlymixed in a ballmill. Thereafter the coating layer 54 was formed on theSUS304L plate by using a hot press so that the average thickness was 20μm. This coating layer 54 was a tight thin film having a volumeresistivity of 15 mΩ-cm.

On the surface of the coating layer 54, 0.5 mm-thick ribs set withcarbon fibers and the epoxy resin were disposed. On the back surface(the cathode-side channel formation face 50C) of the metal plate 52having coating layers 54 formed on its two surfaces, 0.7 mm-thick ribsfor an oxidizer were extruded with a mold at 120° C. and set at 180° C.to form channels for the oxidizer. The through resistivity of the ribswas 5 mΩ-cm or less. The porosity of the porous rib 56 c was about 70%,while the porosity of the gas-permeable rib 56 b was about 50%.

The MEA 40 was made by disposing an anode electrode and a cathodeelectrode made by TKK (the amount of supported Pt: 0.5 mg/cm²) on eachof the two surfaces of a solid electrolyte membrane on the market(Nafion112: trademark) and by using a hot press. The MEA 40 was thensandwiched between a pair of separators 50, thus assembling a unit cell.

The temperature of the unit cell was set to 70° C. and the amount ofhumidification of hydrogen gas to be supplied to the anode side wasadjusted by changing the humidification temperature of a humidifyingdevice. Hydrogen gas (hydrogen utilization rate U_(H2)=70%) and pureoxygen or air (oxygen utilization rate Uair, U_(O2)=40%) wererespectively supplied to the unit cell at ordinary pressure. The powergeneration characteristic when air was used as oxidizer gas is plottedin FIG. 9, as indicated by ▴. In a case where the current density was0.5 A/cm², the initial voltage was 0.68 V. This value is substantiallyequal to or higher by about 10 mV than the voltage of the cell havingribs of the conventional type. It was found that the voltage drop in ahigher-current-density region was smaller and stabler than that in theconventional cell.

Plotting ∇ in FIG. 9 indicates the case of using pure oxygen as oxidizergas. In a lower-current-density region, at a current density of 0.5A/cm², a voltage higher by about 25 mV than that in the case of usingair as oxidizer gas was exhibited due to the high oxygen concentration.In a higher-current-density region where the current density is 0.7A/cm² or higher, however, this power generation characteristic wasreversed and a voltage higher by 0.2 V was obtained at a current densityof 1 A/cm² in the case of using air as oxidizer gas.

In the higher-current-density region, the flow rate of the fuel oroxidizer is reduced because of a larger amount of reaction, and theoperation is in a supply-limited state. Nevertheless, the powergeneration characteristic in the higher-current-density region in thecase of using pure oxygen as oxidizer gas was, for example, a currentdensity of 0.8 A/cm² even in use of the same MEA, and a voltage higherby 50 mV than that in the cell having ribs of the conventional type.

It is therefore thought that the reason that the voltage is stabilizedin the higher-current density region even when air having a low oxygenconcentration is used is because hydrogen gas, humidifying water andoxygen are sufficiently supplied into the catalyst layers in the MEA bypermeating through the porous rib or gas-permeable rib, and because theproduct water is sufficiently removed.

It was also confirmed that even when the amount of humidification wasincreased by increasing stepwise the humidification temperature in therange from 60 to 90° C., the voltage was not substantially changed andthere was no stagnation of water in the MEA 40 face. This is because theflowability of the gas/liquid in the ribs was improved. Further, this isthought since the metal plate and the coating layer were extremely thin(<0.2 mm) and had flat surfaces, high heat conductivity was obtained andheat of condensation produced at the time of movement of anode-sidewater into the electrolyte membrane and heat of reaction produced by thereaction resistance of the cathode were efficiently utilized forevaporation removal of water produced at the cathode.

FIG. 10 show a metallic separator 50 for a direct methanol fuel cell(DMFC) to which the present invention was applied. One face of thismetallic separator 50 is an anode-side channel formation face 50A shownin FIG. 10( a), and the other face is a cathode-side channel formationface 50C shown in FIG. 10( b), thus forming a bipolar structure. FIG.10( c) is a diagram for explaining the flow of air on the cathode sideshown in FIG. 10( b). The members indicated by the same referencecharacters as those in FIG. 8 are the same members or have the samefunctions.

The direct methanol fuel cell is supplied with a methanol aqueoussolution (hereinafter referred to as “methanol fuel”) provided as a fuelinstead of hydrogen gas, at the anode side in Example 1. The electrodereaction at this time is shown below.

Anode: CH₃OH+H₂O→6H⁺+6e ⁻+CO₂

Cathode: 3/2CO₂+6H⁺+6e ⁻→3H₂O  (Formula 2)

Thus, this fuel cell differs from the polymer electrolyte fuel cellshown in FIG. 8 mainly in that a methanol fuel in liquid form issupplied to the anode, and that carbon dioxide gas is produced as areaction product.

The structure of the metallic separator 50 on the anode side in FIG. 10,not particularly illustrated, is substantially the same as that of thepolymer electrolyte fuel cell 100 shown in FIG. 8 except mainly that achannel is formed by a porous rib 56 d on a wide surface adjacent to theMEA, and that the porosity of the porous rib 56 d exceeds 70% (ribs inthe form of rods are formed on the metallic separator 50 on the cathodeside in the same manner as shown in FIG. 8, and no description will bemade of them). As a special example of the channel structure on theanode side, a structure in which the width of the porous rib 56 d isincreased to fill the entire channel as described above may be adopted.

In the anode-side channel formation face 50A, the methanol fuel isforcibly supplied from the fuel supply manifold 55 side by a fluid feedpump, not shown in the figure, or the like. The methanol fuel then flowsin the porous rib 56 d (70%<porosity, carbon paper used as the porousrib 56 d in the present embodiment) to be supplied to the MEA not shownin the figure. The remainder of the methanol fuel and a reaction product(carbon dioxide gas, or the like) are discharged through a dischargeport 57. The methanol fuel is forcibly supplied through the porous rib56 d to be uniformly and efficiently supplied to the entire catalystlayer of the MEA not shown in the figure. The produced carbon dioxidegas is also diffused efficiently in the porous rib 56 d to bedischarged.

The occurrence of a phenomenon in which carbon dioxide gas stops thechannel to impede supply of the methanol fuel as in the conventionalcell is thus prevented and the methanol fuel can be caused to diffuseuniformly over the entire reaction surface. The reaction efficiency cantherefore be improved.

If the MEA size is large, the basic structure constituted by the porousrib 56 d, the fuel supply manifold 55 and the fuel discharge manifold 57may be formed on a plurality of metallic separators. Thus, the fuel canbe uniformly supplied to the entire reaction surface.

In the cathode-side channel formation face 50C, gas-permeable ribs 56′can be used, as in the polymer electrolyte fuel cell 100. In particular,it is thereby ensured that oxygen can be supplied to the catalyst layereven in the portions where the MEA contacts the ribs, and that productwater can be removed with efficiency. Also, an arrangement in which fourchannels are reduced to three channels from the air upstream side to theair downstream side as shown in FIGS. 10( b) and 10(c) is adopted toincrease the flow rate, thereby promoting discharge of product water,condensed water and a reaction product or the like.

Also, a nonporous rib 56 a′ may be provided in the channel on thedownstream side to improve the gas flow rate in the channel on thedownstream side in which the oxygen concentration is reduced, therebyenabling quick removal of water or any other material capable ofstopping the channel. The reaction efficiency can therefore be improved.

Also, the corrosion resistance of a metallic separator can be improvedby applying the metallic separator according to the first embodiment ofthe present invention. Therefore, in the direct methanol fuel cell inparticular, corrosion of the metal plate 52 by an intermediate product(formaldehyde, formic acid, methyl formate, or the like) produced in theprocess of oxidation reaction of the methanol fuel can be limited. Thus,even under such a strict corrosive environment, the metallic separatorcan be used for a long time. Since the metallic separator can be reducedin thickness in comparison with the conventional carbon separator, itenables stacked cells to be further reduced in size. Therefore, theentire size of a battery unit including micro-fuel cells and anauxiliary piece of equipment (such as a fuel or an oxidizer supply pump)can be reduced and the possibility to application of fuel cells toportable appliances can be increased.

While the present example has been described with respect to an examplein which a nonporous rib is used as a portion of a channel, it mayalternatively function as a gasket housing. The channel shape is notlimited to that in the present example. Any other channel such as achannel dotted with ribs in land form, a meandering channel or astraight channel may be formed.

While the present example has been described with respect to an examplein which the fuel in liquid form is methanol, the present invention canalso be applied to fuel cells using other organic fuels (ethanol,isopropanol and the like).

Thus, the first embodiment of the present invention is adopted to enableforming of a channel without stamping and to thereby limitstress-corrosion cracking. Also, a tight coating layer can be formed bythermocompression and, therefore, the corrosion resistance of themetallic separator can also be improved. Further, the physicalproperties and placement of a rib constituting a channel can be freelychanged. Therefore, supply/discharge of a fuel or an oxidizer/productsand the like and supply/dissipation of heat can be uniformized, therebylargely improving the reaction efficiency (power generation efficiency)of the polymer electrolyte fuel cell.

INDUSTRIAL APPLICABILITY

The present invention can be applied to fuel cells which use a polymerelectrolyte, and which are used mainly as power sources for use onvehicles, fixed use (home use), and use in portable appliances such asportable telephones and note PCs.

1. A metallic separator for fuel cells characterized by comprising: ametal plate of austenitic stainless steel; a corrosion-resistant coatinglayer covering at least a surface in front and back surfaces of themetal plate which contacts a raw material and/or a reaction product, andcontaining a carbon-based electroconductive material and a polymerresin; and an electroconductive channel-forming member disposed on asurface of the coating layer and forming a channel for the raw materialand/or the reaction product and/or a channel for a cooling medium forcooling, the metallic separator also being also characterized in that asurface layer on the metal plate has a tensile residual stress withinsuch a range that no stress-corrosion cracking occurs.
 2. The metallicseparator for fuel cells according to claim 1, characterized in that thetensile residual stress in the surface of the metal plate is 15 kg/mm²or less.
 3. (canceled)
 4. The metallic separator for fuel cellsaccording to claim 1, characterized in that the surface of the metalplate is plated with a metal formed of one or more of nickel, gold,silver and platinum.
 5. The metallic separator for fuel cells accordingto claim 1, characterized in that the coating layer and/or thechannel-forming member contains a carbon-based electroconductivematerial or a carbon-based electroconductive material and a polymerresin.
 6. The metallic separator for fuel cells according to claim 1,characterized in that the carbon-based electroconductive materialcontained in the coating layer and the channel-forming member is one ormore of graphite, carbon black, diamond-coated carbon black, siliconcarbide, titanium carbide, carbon fibers and carbon nanotubes.
 7. Themetallic separator for fuel cells according to claim 1, characterized inthat the polymer resin contained in the coating layer and thechannel-forming member is one or more of a phenolic resin, an epoxyresin, a melamine resin, a rubber resin, a furan resin and apolyvinylidene fluoride resin.
 8. The metallic separator for fuel cellsaccording to claim 5, characterized in that the carbon-basedelectroconductive material in the coating layer contains granular carbonand fibrous carbon and the mass ratio of the granular carbon and thefibrous carbon is within the range from 1:0.5 to 1:1.5.
 9. The metallicseparator for fuel cells according to claim 5, characterized in that thecoating layer contains 40 to 65 mass % of the carbon-basedelectroconductive material and the volume resistivity of the coatinglayer is 50 mΩ-cm or less.
 10. (canceled)
 11. metallic separator forfuel cells according to claim 1, characterized in that the coating layeris formed by thermocompression including a hot press or a hot roll. 12.The metallic separator for fuel cells according to claim 11,characterized in that the thickness of the coating layer is within therange from 10 to 100 μm.
 13. The metallic separator for fuel cellsaccording to claim 5, characterized in that the channel-forming membercontains 40 to 80 mass % of the carbon-based electroconductive material.14. The metallic separator for fuel cells according to claim 1,characterized in that the channel-forming member is formed on thecoating layer by injection molding or forming in a mold.
 15. (canceled)16. The metallic separator for fuel cells according to claim 1,characterized in that the channel is formed by combining one or more ofthe channel-forming member having a porosity of 50% or more, thechannel-forming member having a porosity of 10 to 50% and thechannel-forming member having a porosity of 10% or less.
 17. Themetallic separator for fuel cells according to claim 16, characterizedin that the channel-forming member is one or more of porous carbon-basedelectroconductive materials: carbon particle sintered material, carbonfiber sintered material, carbon fiber woven fabric and carbon fibernonwoven fabric, and the channel-forming member is joined to the coatinglayer. 18-27. (canceled)
 28. A method of manufacturing the metallicseparator for fuel cells according to claim 1, characterized by formingthe channel for the raw material and/or the reaction product and/or thechannel for the cooling medium for cooling by forming thecorrosion-resistant coating layer containing a carbon-basedelectroconductive material and a polymer resin on at least a surface inthe front and back surfaces of the metal plate of austenitic stainlesssteel which contacts the raw material and/or the reaction product, andby thereafter disposing the one or more electroconductivechannel-forming members on the surface of the coating layer.
 29. Themethod of manufacturing the metallic separator for fuel cells accordingto claim 28, characterized in that the coating layer is formed byadhering a coating layer forming liquid, a coating layer forming powderor a coating layer forming sheet containing a carbon-basedelectroconductive material and a polymer resin to the surface of themetal plate by thermocompression including a hot press or a hot roller.30. The method of manufacturing the metallic separator for fuel cellsaccording to claim 28, characterized in that the channel-forming memberis formed by forming a channel forming liquid or a channel formingpowder containing a carbon-based electroconductive material and apolymer resin on the coating layer by injection molding or forming in amold to provide the channel for the raw material and/or the reactionproduct and/or the channel for the cooling medium for cooling. 31-32.(canceled)
 33. A metallic separator for fuel cells characterized bycomprising: a metal plate; an electroconductive coating layer coveringat least a surface in front and back surfaces of the metal plate whichcontacts a raw material and/or a reaction product; and anelectroconductive channel-forming member disposed on a surface of thecoating layer, containing one or more of porous carbon-basedelectroconductive materials: carbon particle sintered material, carbonfiber sintered material, carbon fiber woven fabric, and carbon fibernonwoven fabric, and forming a channel for the raw material and/or thereaction product and/or a channel for a cooling medium for cooling, themetallic separator also being characterized in that the channel isformed by combining one or more of the channel-forming members having aporosity of 50% or more, the channel-forming members having a porosityof 10 to 50%, and the channel-forming members having a porosity of 10%or less, and a surface layer on the metal plate has a tensile residualstress within such a range that no stress-corrosion cracking occurs. 34.(canceled)
 35. The metallic separator for fuel cells according to claim33, characterized in that the metal plate is austenitic stainless steelplate.
 36. The metallic separator for fuel cells according to claim 33,characterized in that the tensile residual stress in the surface of themetal plate is 15 kg/mm² or less.
 37. The metallic separator for fuelcells according to claim 33, characterized in that the coating layercontains a carbon-based electroconductive material and a polymer resin.38. A method of manufacturing a metallic separator for fuel cellscharacterized by comprising: a step of forming an electroconductivecoating layer at least on a surface in front and back surfaces of themetal plate which contacts a raw material and/or a reaction product; anda step of joining to the surface of the coating layer anelectroconductive channel-forming member containing one or more ofporous carbon-based electroconductive materials: carbon particlesintered material, carbon fiber sintered material, carbon fiber wovenfabric, and carbon fiber nonwoven fabric, and forming a channel for theraw material and/or the reaction product and/or a channel for a coolingmedium for cooling, the method also being characterized in that thechannel is formed by combining one or more of the channel-formingmembers having a porosity of 50% or more, the channel-forming membershaving a porosity of 10 to 50%, and the channel-forming members having aporosity of 10% or less, and in that a surface layer on the metal platehas a tensile residual stress within such a range that nostress-corrosion cracking occurs.
 39. The method of manufacturing themetallic separator for fuel cells according to claim 38, characterizedin that the coating layer is formed by adhering a coating layer formingliquid, a coating layer forming powder, or a coating layer forming sheetcontaining a carbon-based electroconductive material and a polymer resinto the surface of the metal plate by thermocompression including a hotpress or a hot roller.
 40. The method of manufacturing the metallicseparator for fuel cells according to claim 38, characterized in thatthe channel-forming member is formed by forming a channel forming liquidor a channel forming powder containing the porous carbon-basedelectroconductive material and the polymer resin on the coating layer byinjection molding or forming in a mold.
 41. A fuel cell constructed bysandwiching a cell having a cathode catalyst layer on one surface of apolymer electrolyte membrane and an anode catalyst layer on the othersurface of the polymer electrolyte membrane between a first separator inwhich a channel for flowing a fuel to the anode catalyst layer is formedand a second separator in which a channel for flowing an oxidizer to thecathode catalyst layer is formed, the fuel cell being characterized inthat the metallic separator for fuel cells according to claim 1 is usedas each of the first separator and the second separator.